The present invention relates generally to audio power amplifiers. More particularly, this invention pertains to a system and method for protecting an audio amplifier output stage power transistor.
Audio amplifiers are well known in the art and, as the name suggests, are used to amplify audio signals. These amplifiers typically include an audio input, which is connected to some type of audio source, and an audio output, which is connected to audio speakers. These amplifiers receive audio signals from the audio source, amplify those signals, generate audio current signals based on those amplified signals, and output the current signals to the speakers. It is these current signals that drive the speakers and cause them to reproduce the audio signals that are generated by the audio source to create sound.
To generate the current signals necessary to drive the speakers, audio amplifiers typically include an output stage specifically designed for that purpose. The output stage usually includes a pair of power transistors that are coupled to one another and a power source, which provides power to the power transistors so they can generate the necessary output currents. Each power transistor supplies a predetermined portion of the required output current.
There are a variety of different types of audio amplifier output stages in the prior art. There are Class A output stages, which include power transistors that operate continuously, and Class B output stages, which include power transistors that operate only 50% of the time. There are also Class AB output stages, which include power transistors that operate somewhere between 50% of the time and continuously, and Class C output stages that include power transistors that operate less than 50% of the time.
The following discussion will focus specifically on Class B type audio amplifier output stages. It is noted, however, that Class A, Class AB, Class G, Class H, and other types of output stages known in the art, suffer from problems similar to the ones discussed below with regard to Class B output stages. As a result, the solution to the problems with Class B output stages presented in this application may be similarly applied to solve the problems associated with these other types of output stages as well.
Class B audio amplifier output stages typically include two bipolar power transistors directly coupled to one another and a power supply. The power supply is connected to the power transistors and supplies the transistors with the power necessary to generate the required output currents. A portion of a typical Class B audio amplifier output stage is shown in FIG. 1. In that figure, Q1out is a NPN bipolar power transistor, biased to operate as a Class B transistor, and Q2out is a PNP bipolar power transistor, also biased to operate as a Class B transistor. The power supply is shown as + and −Vrail. For convenience, other components that are included with these types of output stages are not shown in FIG. 1. In addition, the biasing circuits necessary to properly bias the transistors, which are well known in the art, are not shown in FIG. 1.
The power transistors used in the output stage of the audio amplifier shown in FIG. 1 can only be operated under certain conditions or they will fail. Power transistors such as these have specific bond wire limits, power dissipation limits, breakdown voltage limits, and second voltage breakdown limits. If any of these limits are exceeded, the power transistors can fail.
The combination of these limits generates something that is commonly referred to as a power transistor's Safe Operating Area, or SOA. As shown in FIG. 2, the SOA for a power transistor is generally defined in terms of the voltage across the transistor, Vce, and the current passing through the transistor, Ice. As long as the voltage across and the current passing through a power transistor stay within certain limits, the transistor will operate properly. If these parameters are exceeded for too long, however, the transistor will fail.
It is important to note that the above-referenced SOA defines the power transistor's steady state operating limits. As explained in detail in this application, these limits may be exceeded for limited time periods without damaging the power transistor. This is true for two reasons. First, power transistors typically fail because of excessive heating, which is, in turn, caused by excessive voltage and current. Second, power transistors have thermal impedance and, as a result, it takes a certain amount of time for the transistor to heat up to the point where it is damaged. This aspect of power transistor operation and the potential benefits associate with allowing the power transistor to temporarily exceed these limits are discussed in more detail below.
Looking at FIG. 2, the x-axis, or horizontal axis, is the voltage applied to the transistor, Vce, and the y-axis, or the vertical axis, is the current passing through the transistor, Ice. Line a, which corresponds to the steady state operation of a power transistor at 25 degrees Celsius, defines the SOA for the power transistor. The area beneath line a is the SOA and the area above the SOA is considered to be outside of the SOA. If the transistor is operated above the SOA for a predetermined time period, the transistor will fail.
Lines b also defines a SOA for the power transistor. Line b defines the SOA for the steady state operation of the power transistor at 100 degrees Celsius, Line c defines the load line when the power transistor is driving a resistive load, and Line d defines the load line when the power transistor is driving a reactive load. As before, the SOA is defined as the area beneath each line and the power transistor will fail if operated in the areas above the lines for too long.
A power transistor may operate outside of the SOA for many reasons. This typically occurs when one of the power transistor circuits develops a short circuit and causes the power transistors to deliver more current than they are rated to deliver. The power supplies used with power transistors are able to provide more current to a power transistor than it can handle without causing damage and, when a short circuit occurs, they do exactly that.
The prior art has addressed this problem using various different types of audio amplifier output stage protection circuits, or simply, protection circuits. The prior art includes simple current limiter protection circuits that limit the output current of a power transistor to a predetermined maximum value. An example of a prior art current limiter protection circuit is shown in FIG. 3 and a plot showing the protection threshold (line e) generated by that circuit is shown in FIG. 4.
The current limiter prevents the transistor from delivering too much current when a short circuit occurs and destroying itself. The operation of current limiter circuits is well known in the prior art. However, a brief explanation of the operation of these circuits is in order to understand the significance of the present invention.
As shown in FIG. 3, prior art current limiters typically include a resistor network that is connected to the output of the power transistor and to a second transistor, which is, in turn, connected to the base of the power transistor. These circuits also typically include a diode that is connected between the second transistor and the power transistor base.
In FIG. 3, the power transistor is Qout, the resistor network includes RE, R1, and R2, the second transistor is Q1, and the diode is D1. Vout represents the voltage output of the power transistor and is connected to a speaker (not shown) or some other appropriate load. Vce is the voltage across Qout, Ice is the current flowing through Qout, Ib is the base current flowing into Qout, and Vbe is the emitter-base voltage for Q1. Also, the second power transistor that is typically included in an audio amplifier output stage is not shown in FIG. 3 in order to simplify this discussion.
RE is much smaller than both R1 and R2. The small resistance of RE, and the large resistance of R1 and R2, causes the majority of Ice to flow through RE to the speaker. Only a small amount of Ice is diverted to R1 and R2 and used to operate the protection circuit. The diode D1 prevents current from flowing back into the circuit that generates the base signal Ib.
The resistor network of RE, R1, and R2 is a voltage divider network. As a result, when voltage builds up on RE, proportional voltages build up on R1 and R2. When the voltage on R2 builds up to a predetermined level, Vbe for Q1, which in this case is approximately 0.6 to 0.7 volts, Q1 turns on and begins diverting base current Ib away from the base of Qout. This, in turn, prevents Ice for Qout from increasing any further.
Under normal operating conditions, only a small amount of voltage builds up on RE. This voltage is not sufficient to generate a voltage on R2 that will turn Q1 on. If a short circuit occurs, however, Ice will increase significantly and generate a large voltage on RE. This voltage is transferred through the voltage divider network to R2 and, when the voltage across R2 reaches 0.6 to 0.7 volts, Q1 turns on and prevents Ice from increasing further. Consequently, the power transistor is prevented from destroying itself by supplying a large output current.
While current limiter circuits do limit the amount of current output by power transistors, they also interfere with the normal operation of the very power transistors they are designed to protect. As mentioned previously, the protection threshold provided by a typical current limiter is shown in FIG. 4. Line e represents the protection limit provided by the current limiter and illustrates how the protection circuit prevents the transistor current, Ice, from exceeding a predetermined limit.
Unfortunately, the protection circuit also prevents the transistor from operating in portions of the SOAs associated with lines a and b. In other words, the current limiter prevents the power transistor from operating in areas where it can safety operate.
The prior art also includes voltage-current limiter protection circuits, commonly referred to as VI limiters, which limit the output current and output voltage of a power transistor to predetermined maximum values. VI limiter protection circuits include single-slope VI limiters and two-slope VI limiters. Single slope VI limiters, as their named suggests, provide a protection threshold that forms a sloping straight line. In a similar manner, two slope VI limiters provide a protection threshold that forms a line having two different slopes. Examples of single slope and two slope VI limiters are shown in FIGS. 5 and 6, respectively. Plots showing examples of the protection thresholds (line e in each figure) generated by these types of circuits are shown in FIGS. 7 and 8, respectively.
VI limiters provide protection to the output stage by limiting the power transistor current, Ice, and the power transistor voltage, Vce. As shown in FIG. 7, this type of protection circuit generates a sloping protection threshold, line e, rather than the horizontal line (line e in FIG. 4) generated by the simple current limiter protection circuit.
A typical VI limiter protection circuit (FIG. 5) is very similar to the simple current limiter discussed previously. The only difference between the two types of circuits is the addition of a resister, R3, to the resister divider network. The resistor, R3, is connected to the power supply Vrail. R3 has a much larger resistance than R1 and R2, and RE has a resistance that is much smaller than R1, R2, and R3.
The operation of the VI limiter is also very similar to the simple current limiter protection circuit. With regard to the power transistor current, Ice, the circuits operate exactly the same. If Ice increases, a voltage is generated on RE and transferred onto R2. When the voltage on R2 reaches Vbe, Q1 turns on and begins diverting base current from Qout and prevents Ice from increasing further. The difference between the two circuits, however, relates to how the circuits respond to the voltage, Vce, developed across the power transistor.
The addition of the resister R3 modifies the voltage divider network so an additional voltage is generated across R2. The power supply voltage Vrail drops part of its voltage across R3 and part of its voltage across the parallel combination of R2 and R1 plus RE.
During normal operations, the voltage developed across R2 is insufficient to cause Q1 to turn on, i.e., the voltage across R2 is less than Vbe for Q1. When the power transistor voltage Vce increases, however, the voltage across R2 also increases. If Vce increases to a certain predetermined level, the voltage developed across R2 will reach Vbe for Q1 and Q1 will turn on. Once this happens, Q1 diverts base current Ib from Qout and prevents Vce from increasing further.
As was the case with the simple current limiter, the VI limiter circuit also interferes with the normal operation of the power transistors it is designed to protect. Referring back to FIG. 7, the VI limiter protection threshold is a sloping line, line e, that extends from Ilim1 to Vlim. A power transistor connected to this type of protection circuit cannot operate in the area above line e and this prevents the transistor from operating in a portion of its SOA. Once again, this type of operation is undesirable for the same reason as that discussed above for the simple current limiter.
As mentioned above, a typical two slope VI limiter protection circuit is shown in FIG. 6 and the typical protection threshold for that circuit (line e) is shown in FIG. 8. A review of this protection circuit indicates that it is very similar to the single slope VI limiter discussed previously. The difference between the two circuits is the addition of a resistor R4 and a diode, D2, which is tied to ground, to the resistor voltage divider network. When Vce for Qout is less than or equal to Vrail, the diode D2 turns on and R4 becomes part of the voltage divider network. When Vce exceeds Vrail, D2 turns off and R4 is taken out of the network.
The use of R4 causes the slope of the protection threshold, line e, to change. This change in slope, in turn, causes Vlim to increase. The increase in Vlim, in turn, allows the power transistor to operate in more of its SOA. Although the two slope VI limiter protection circuit is an improvement over the single slope VI limiter and the simple current limiter, it still interferes with the normal operation of the power transistor it is designed to protect.
While all of the prior art protection circuits discussed above do protect the power transistor when properly used, they also unnecessarily interfere with the normal operation of the power transistor. This occurs for a number of reasons. As seen in FIG. 2, the operating characteristics of a power transistor vary nonlinearly in response to changes in the power transistor voltage and current. The protection thresholds generated by the prior art protection circuits, however, vary linearly. As a result, the protection thresholds do not accurately track the operating characteristics of the power transistor and prevent the power transistor from operating in areas it normally would be able to operate. This characteristic of a simple current limiting protection circuit can be seen in FIG. 4, which shows the protection threshold, line e, limiting the power transistor output current so that the power transistor cannot operate in areas that are clearly below lines a, b, c, and d and, therefore, within the normal operating ranges of the power transistor. Similar limiting characteristics of single slope and two slope VI limiters can be seen in FIGS. 7 and 8.
The operating characteristics of a power transistor also vary in response to the operating temperature of the power transistor. As the temperature increases, the operating limits decrease. As the temperature decreases, the operating limits increase. Protection thresholds generated by prior art protection circuits do not vary with the power transistor temperature. Thus, prior art protection circuits interfere with the normal operation of power transistors for this reason as well. This characteristic of a power transistor can clearly be seen in FIGS. 4, 7, and 8, which each show the operating characteristics of a power transistor at 25 degrees Celsius, line a, having higher operating limits than the operating characteristics of the power transistor at 100 degrees Celsius, line b. The failure of the protection thresholds generated by the prior art protection circuits to vary with temperature is also shown in these figures.
Regardless of the reason, the interference with normal power transistor operation is an undesirable condition. The purpose of the power transistor is to generate an output current that is representative of the audio input signal and that is capable of driving a speaker so that it reproduces the sounds associated with the audio input signal. When the prior art protection circuits limit a power transistor as discussed above, the power transistor can no longer generate an output current that is representative of the audio input signal and, as a result, the speaker no longer accurately reproduces the sounds associated with the audio input signal.
A solution to the problems presented by these prior art protection circuits is taught in U.S. Pat. No. 4,330,809, issued to Stanley on May 18, 1982 and entitled “Thermal Protection Circuit For The Die Of A Transistor.” The '809 patent teaches a protection circuit for an audio amplifier output stage power transistor that generates a protection threshold that more accurately tracks the output capabilities of a power transistor. In other words, the circuit provides protection when necessary without interfering with the normal operation of the power transistor.
Stanley accomplishes this task by using a protection circuit that generates a signal that is representative of the power applied to a power transistor, transforms this signal into a signal that is representative of a temperature differential between the power transistor die and heat sink, senses the temperature of the power transistor heat sink, and combines the temperature differential signal with the sensed heat sink signal to generate a signal representative of the temperature of the power transistor die. The power transistor die is the portion of a power transistor that can be damaged by excessive heat and cause the power transistor to fail. The Stanley protection circuit uses the die temperature signal, in conjunction with a control circuit, to reduce the power dissipation of the power transistor if a predetermined die temperature is exceeded.
The protection circuit taught in the '809 patent, however, is a complex circuit and can be difficult to implement in practice. As shown in FIG. 2 of the '809 patent, the protection circuit requires several different types of circuit components in order to operate properly, including a differential pair of matched transistors, which form a multiplier, two operational amplifier circuits, an RC feedback network, and a temperature sensor for sensing the temperature of the heat sink used with the power transistor. As a result, this protection is undesirable in some applications.
What is needed, then, is a system and method for protecting an audio amplifier output stage that is less complex and easier to implement than the protection circuit disclosed in the '809 patent and that protects a power transistor without interfering with the normal operation of that transistor.